The present invention relates to a process for winding a fiber material into a plurality of wrappings and to a winding device useful for the process. The winding device has a rotational axis and two side walls.
Energy absorbing beams such as leaf springs are useful in a great number of applications, mainly in the automobile industry.
Presently multiple-leaf springs made of metals are still being used in the industry although the advantages of fiber reinforced plastic leaf springs such as much lighter weight, longer life and no corrosion problems have been known for several years, see the literature cited in U.S. Pat. No. 4,414,049. The best fiber reinforced plastic leaf springs presently known are the so-called parabolic springs which have a curved shape in the longitudinal direction and a crosssection decreasing in the longitudinal direction of the spring from the central part towards the ends along at least a substantial part of this length. Such leaf springs can absorb energy evenly and have an excellent strength-to-weight ratio.
The extended use of metallic leaf springs is mainly due to the difficulty of producing such a fiber reinforeed plastic leaf spring in an economical way.
Known processes for producing fiber reinforced plastic leaf springs are:
(1) prepreg/laminate compression moulding and
(2) filament winding.
According to the first process preimpregnated filaments, cured laminates or layers of dry or wet fabrics are placed into a mould. However, this method is labor-intensive and therefore costly.
Therefore, processes for filament winding have been suggested.
In European published patent application 0 005 916 a method of producing fiber reinforced plastic leaf springs is disclosed wherein layers of fibers are wound onto an elliptical surface of a former by rotating the former about an axis. When a sufficient quantity of fibers are built upon the former, spacer elements pre-formed from randomly oriented fibers in a resin matrix are applied to the former. Further layers of fibers are then built up by further winding on the former. The resulting assembly is then cut at the ends of the former. This gives two individual part-elliptic leaf springs. The produced spring has first and second layers of fibers which are oriented to lie longitudinally of the spring and between them a spacer element preformed from randomly oriented fibers in a resin matrix. Unfortunately, the spacer element is not able to absorb much energy. The load-bearing portion of the leaf spring, i.e. the fibers which are longitudinally oriented to the spring, has a constant thickness over the entire length of the spring. This decreases the ability of the spring to absorb energy. Furthermore, the use of several materials having different physical properties may cause delamination when the spring is under stress.
In U.S. Pat. No. 4,414,049 a method for producing fiber reinforced plastic leaf springs is disclosed wherein a plurality of resin impregnated filaments are wound into a continuous band of side-by-side filament strands. The side-by-side strands are wrapped about a pattern formed on a base by a plurality of pins whose number is increased as the side-by-side strands are wrapped about the pins. The base into which the pins are inserted is rotated. When wrapping is completed, the thickness of the beam formed is greatest in the area established by the first set of pins and least in the area established by the last set of pins. After the wrapping has been completed, the strands are cut and folded into the desired position: the wrapped strands are then placed into a mold which is closed and exposed to heat and pressure for curing the filament reinforced plastic.
Although leaf springs having the desired shape can be obtained by this process, it is still not economical. The rotating base has to be stopped every time that additional pins are inserted.
European published patent application 0 200 076 discloses a similar winding process. The winding device comprises an axis of rotation and two end walls which are oriented perpendicular to the axis of rotation and arranged in spaced relationship with each other. Each end wall is provided with a plurality of holes through which pins are insertable. Fiber material is wound around the pins. The disclosed process is based on the same principle as the process in U.S. Pat. No. 4,414,049 and has unfortunately the same disadvantages. The winding has to be interrupted every time when additional pins are to be inserted which renders the process slow and uneconomical.
The English Abstract of JP-A-56 141 435 discloses a method of producing a fiber reinforced plastic leaf spring by repeatedly folding a fibrous cloth tape in the longitudinal direction and continuously piling up the portions of the tape. The fibrous cloth has been preimpregnated with a synthetic resin. The drawings of JP-A-56 141 435 illustrate that the process of repeatedly folding the fibrous cloth tape is based on the same principle as the processes disclosed in U.S. Pat. No. 4,414,049 and European published patent application 0 200 076 and consequently has the same disadvantages.